Lubricating oils are required to have various performances in accordance with their use purposes. In particular, engine oils are required to have high thermal stability, high-temperature detergency, oxidization stability, wear prevention and others, and are produced by incorporating lubricating oil additives such as an anti-wear agent, an ashless dispersing agent, a metal detergent, and an antioxidant thereinto. Examples of the metal detergent include such as salicylates, phenates and sulfonates. These are used alone or in combination in order to improve the high-temperature detergency and other properties of lubricating oils.
About such a technical field, for example, patent document 1 (Japanese Patent Application Laid-Open (JP-A) No. 8-176583) and patent document 2 (JP-A No. 10-53784) disclose diesel engine oil compositions into which a combination of metal detergents having different base numbers is incorporated.
However, the following have been coming to light: in the case of using a combination of a monoalkyl salicylate and a metal detergent (such as a sulfonate) other than any salicylate both of which have been in general commercially available and used hitherto, calcium carbonate and others, which are dispersed in the metal detergent, precipitate when the composition is stored, so as to cause the following problems: the clogging of a producing line filter, a shipping line filter, an engine filter and so on for the lubricating oil additives and the lubricating oil; a drop in qualities required as products of the lubricating oil additives or the lubricating oil, such as a drop in the base number thereof; the generation of abnormal abrasion when the composition is actually used; and others. In particular, in the case of using a monoalkyl salicylate made into a (per)base by use of calcium carbonate, calcium borate or the like together with a neutral or (per)basic sulfonate detergent, in particular, a neutral sulfonate, precipitation is generated at an early stage. Thus, this combination cannot be virtually used in any lubricating oil additive or lubricating oil product under the present circumstances. Thus, the circumstances have been desired to be improved.